To keep articles in sanitary condition, antimicrobial substances are applied to their surface under certain circumstances. For example, in food-handling facilities, domestic kitchens and medical facilities, it is required to keep the surface of the interior floor and walls, instruments, fixtures or other articles germ-free in order to prevent possible contact infection or poisoning caused by harmful microorganisms such as bacterial pathogens attached to their surface. It is also required to effectively prevent mold growth or food spoilage in such places as house closets or food depositories, which are highly humid and less ventilated places, part of a building where water is circulated and thus condensation frequently forms (e.g., bathroom), and the inside of an air-conditioner or refrigerator. Thus, it has been required to impart a harmful microorganism reduction effect to the surface of articles such as building materials (e.g., wall paper), food storage containers, and bathroom accessories.
One widespread method of imparting a harmful microorganism reduction effect to the article surface is to incorporate therein an antimicrobial agent or apply thereon antimicrobial agent-containing coating. Antimicrobial agents are classified into organic and inorganic antimicrobial agents. In particular, inorganic antimicrobial agents such as silver ion-bearing zeolites are attracting attention because they have a wider spectrum of activity to harmful microorganisms than organic counterparts as well as are less toxic to the human body.
Moreover, some types of metals are known as a substance that has antimicrobial activity; in particular, silver, copper and alloys thereof are known. For preventing food poisoning and mold growth, these metals have been widely used for eating utensils, wash-basin, building materials, etc. Attempts have also been made to make use of the antimicrobial effect of copper and copper alloys for infection control medical facilities or other places (see Non-Patent Literatures 1 to 8). In fact, in some medical facilities, copper and copper alloys are used for metallic parts (e.g., doors), bed fences, etc.
However, articles having an organic antimicrobial agent incorporated therein or applied thereon do not necessarily have a satisfactory harmful microorganism reduction effect because the antimicrobial agent shows antimicrobial activity to only limited strains of microorganisms when used singly, because new strains of resistant microorganisms are more likely to emerge, and so forth.
Inorganic antimicrobial agents have a relatively superior effect of reducing harmful microorganism. However, in the case of articles prepared by incorporating inorganic antimicrobial agents into polymer materials, the antimicrobial agent is not necessarily exposed to the entire surface of the article, and therefore, there is a risk that a population of harmful microorganism attached to non-exposed part of the article is not affected by the agent and survives.
Examples of articles prepared by processing of an antimicrobial metal (e.g., silver or copper) itself include copper-made garbage boxes for sink (so-called triangular sink tidy) and brass fittings. Because these articles have disinfecting activity over the entire surface, there is no concern that such a case as described above—where a population of harmful microorganism present on the surface free from inorganic antimicrobial agent is not affected by the agent—will occur. Nevertheless, due to their high specific gravity, the obtained article becomes not only heavy, but costly. Moreover, articles made of silver or copper have the disadvantage of being susceptible to discoloration by contact with water, acid or salt, which makes their appearance poor.
Aiming to suppress such discoloration in metals, studies have been made on corrosion-resistant alloys, including bronze, which is composed primarily of copper with tin as the additive, aluminum bronze, which is composed primarily of copper and aluminum as the additive, and nickel silver, which is composed primarily of copper with nickel and zinc as the additives. It has been scientifically verified that some of the corrosion-resistant alloys have an antimicrobial effect (see Non-Patent Literatures 1 to 6). However, they are not widely used in view of production costs, weight, and processability.
For example, while bronze, a metal alloy composed primarily of copper with tin as the additive, is known to have higher corrosion resistance with increasing tin proportion, it becomes more brittle as the tin proportion increases and thus becomes less processable. For this reason, in the case of a plate or other form that requires subsequent processing, the highest possible tin proportion is about 5 wt % (about 3 atom %) based on copper, and in the case of a cast that requires no subsequent processing, the highest possible tin proportion is about 10 wt % (about 6 atom %) based on copper. Thus, there is limitation in increasing corrosion resistance. On the other hand, aluminum bronze (metal alloy composed primarily of copper and aluminum as the additive) and nickel silver (metal alloy composed primarily of copper with nickel and zinc as the additives) have superior corrosion resistance, but they are not widely used due to their relatively high production costs and high specific gravity of 8 or more.
In particular, as for products having on their surface an antimicrobial thin metal film, poor appearance is a serious problem. The surface of bulk copper or copper alloy is covered with a relatively stable oxide film with a thickness of the order of submicrons, which suppresses the progression of corrosion on their surface. On the other hand, articles having an antimicrobial thin metal film with a thickness comparable to that of such an oxide film cannot be stably fixed to a substrate article because the antimicrobial thin metal film undergoes rapid oxidation over the entire surface. Thus, copper or copper alloy thin films significantly degrade by contact with water compared to bulk copper or copper alloy. In particular, since a pure copper thin film is lost in a relatively short period of time by oxidization, when water droplets remain attached to part of an article covered with such a pure copper thin film, product quality significantly decreases due to the creation of yellow brown spots at the part.
Silver thin films, on the other hand, undergo oxidization sufficiently slowly upon contact with water and therefore raise no problem for practical applications. However, since silver films produce water-soluble silver chlorides upon contact with salts, they have the disadvantage that they undergo discoloration or are lost upon contact with sweat or other body fluid as with thin copper films, which significantly makes their appearance poor.
Various studies have been made on copper alloy thin films. For example, paper and plastic films having on their surface a metal thin film made of, for example, copper, silver or alloy containing copper and/or silver as an antimicrobial metal thin film are proposed (see Patent Literatures 1 to 7). In particular, as an antimicrobial metal thin film to be provided on substrates, a Sn—Cu alloy thin film containing Sn—Cu alloy and 1 to 10 wt % of SnO2 is proposed (see Patent Literature 7). However, none of Patent Literatures 1 to 7 study a metal thin film that can suppress corrosion by water or salt without reducing antimicrobial activity, nor do they disclose a Sn amount that can realize such a film.
On the other hand, as an antimicrobial metal with improved corrosion resistance, an amorphous alloy is proposed which contains 15 atom % to 30 atom % of Ta, 15 atom % to 40 atom % of Cu, 20 atom % to 51 atom % of Fe, 2 atom % to 5 atom % of Ni, and 6 atom % to 14 atom % of Cr (see Patent Literature 8). As an amorphous alloy having antimicrobial activity, oxidization resistance, discoloration resistance and corrosion resistance, an alloy is proposed that contains 5 atom % or more of Ta and/or 15 atom % or more of Nb, as well as Ti and Ni, with the remaining portion substantially consisting of Cu (see Patent Literature 9).
These alloy thin films are formed by a variety of deposition methods. In general, when forming a metal thin film on a substrate like a plastic substrate, metal thin films made of pure metal (e.g., copper or silver) can be produced with relatively good productivity by vacuum deposition. It is difficult, however, to form alloy thin films containing two or more different metals by vacuum deposition. Thus, alloy thin films are typically manufactured for example by flash deposition—a relatively high-cost deposition process, by simultaneous vapor deposition—a deposition process in which two or more separate deposition sources are heated for deposition while being independently controlled, or by sputtering—a less productive deposition process.
It should be noted, however, that it has been suggested in the art that some combinations of metals can be evaporated to deposit an alloy thin metal film even when an alloy is employed as a single deposition source. For example, it has been proposed in the art to deposit a copper-tin alloy on a transparent substrate by heating of a 1:1 ratio copper-tin alloy on a molybdenum board (see Patent Literature 10). Moreover, aiming to improve corrosion resistance of a reflective metal film, it has also been proposed in the art to cover the reflective metal film with a deposited alloy film composed primarily of copper and tin (see Patent Literature 11).